Many radio broadcasting transmitters are equipped with modern temperature controlled crystal controlled oscillators so that these sources of alternating current of constant high frequency would be readily available for use in laboratory measurements were it not for the presence of modulation by program material. Two methods for removing this modulation are described. The first employs an oscillator type circuit that is actuated by the incoming radio signal. It has the virtue of simplicity but is not stable in operation. The second method employs a demodulating amplifier that delivers to a low loss tuned circuit pulses of energy that are almost independent of the modulation. This circuit is stable and does not require adjustment after once having been put in operation on a signal of given frequency. Distorted wave forms appear only when the radio broadcast signal carries modulation approaching or exceeding 100 percent.

Attention is directed toward the difference between induction‐coercive force and magnetization‐coercive force. Although the former is used more commonly the latter appears to have a more fundamental significance, so that a consideration of its meaning has led to a rapid, direct method for determining coercive force. An apparatus is described with which this method is used to determine rapidly the coercive force of solid and granular samples. Advantages of this method and apparatus are discussed.

The theory of magnetic field production by a limited number of circular current loops with a common axis is briefly restated. The previously known particular solutions for two, four (or three), six and eight loops are recomputed and some new solutions of special interest are given. Expansions of surface zonal harmonics and of their first derivatives, up to the twelfth order, are included.